Acute pyelonephritis is an invasive infection that represents a leading cause of bacteremia. Despite progress in understanding the molecular basis of adhesion by organisms that cause pyelonephritis, little is known about how bacteria breach the uroepithelial barrier and invade the renal parenchyma. We have found that human renal epithelial cells can internalize E. coli in a manner similar to phagocytosis and propose that this phenomenon is important in pathogenesis. Furthermore, several lines of evidence suggest that human renal epithelial cells may behave like amateur phagocytes, recognizing bacteria, responding to their presence, ingesting them and perhaps even killing them. Although the efficiency with which the cells out these processes may not approach that of professional phagocytes, the strategic location of epithelial cells at the frontier of the interaction with pathogens and the large number of such cells suggests that they could play an important initial role in containing infection. Alternatively, and perhaps additionally, the uptake of bacteria by renal epithelial cells may provide the means for E. coli to breach the epithelial barrier, gain access to deeper tissues and cause disease. We plan to pursue these studies further by achieving three specific aims; (1) to investigate the fate of intracellular E. coli with regard to their intracellular location and their ability to survive inside cells; (2) to identify signal transduction pathways including protein tyrosine kinase activation associated with internalization of E. coli into human renal epithelial cells; and (3) to identify proteins that are differentially expressed in human renal cells infected with E. coli in comparison to uninfected cells. We believe that the studies described in this proposal will provide important information that will not only increase our understanding of the pathogenesis of pyelonephritis, but may lead to new insights into the cellular biology of host-pathogen interactions.